The lumen method is the best known of the
flux design methods and has the great virtue of simplicity. The basic formula is usually
expressed as follows:

(1)

where E = average illuminance

N
= number of lamps used in the installation

L
= lighting design lamp lumens (the product of initial

lumens
and lamp depreciation factor)

MF
= maintenance factor

AL
= factor to represent atmospheric absorption losses

UF
= utilisation factor of floodlights

A
= area to be lit by floodlights

4.1 Maintenance Factor

Maintenance factor allows the
effect of loss of light due to dirt between cleaning intervals. For a
18-month cleaning cycle in a fairly clean environment, the maintenance factor is either
0.6 or 0.7, depending on the cleaning characteristics. It is much better to
design for a maintenance factor of between 0.9 and 1.0 by choosing floodlights with good
cleaning characteristics and accepting short cleaning cycles.

4.2 Atmospheric Loss

The atmospheric loss is caused by air-borne
moisture and solid particles. The loss varies with:

(a) time of the day,

(b) season,

(c) location,

(d) mounting height, and

(e) length of throw.

In an urban football installation using
four 30 m to 45 m floodlight towers, a typical loss of illuminance on a clear night is as
much as 20 to 30 %.

Lumen calculations should be based on the
performance on a clear night, as the effects of other weather conditions are too
changeable to be taken into account.

4.3 Utilisation Factor

Utilisation factor is a calculated
ratio of the lumens effectively lighting an area to the total available lumens from the
lamp. Table of utilisation foctors are not usually published for floodlight projectors so
it is a common practice to use the zonal flux diagram to determine the
utisation factor of floodlight.